1. Technical Field
The present invention relates to an engine control apparatus for use in a work vehicle.
2. Related Art
When the work vehicle is a bulldozer, engine output (torque) is distributed to a traveling load, work machine load, and cooling fan load through a PTO shaft. This means that the engine output (torque) is transmitted to a sprocket wheel through a traveling power train (power transmission) such as a torque converter and a transmission (hydraulic clutch), whereby crawler belts are driven and the vehicle is caused to travel. Thus, a portion of the engine horsepower is consumed as traveling horsepower (horsepower absorbed by the torque converter). The traveling horsepower input to the traveling power train must be suppressed to a certain horsepower level or lower in consideration of durability of the traveling power train.
The engine output is also transmitted to a work machine hydraulic pump to drive the work machine hydraulic pump. Pressurized oil is thereby supplied from the work machine hydraulic pump to a work machine actuator (e.g., a hydraulic cylinder or a hydraulic motor) to activate the work machine (such as a blade), whereby a work operation is performed. Thus, a portion of the engine horsepower is consumed as working horsepower (horsepower absorbed by the work machine pump).
The engine output is also transmitted to a fan hydraulic pump to drive the fan hydraulic pump. Pressurized oil is thereby supplied from the fan hydraulic pump to a fan hydraulic motor, whereby the rotation of a cooling fan is activated and coolant is held at a desired target temperature. Thus, part of the engine horsepower is consumed as fan horsepower (horsepower absorbed by the fan hydraulic pump).
Accordingly, the following relationship is established:Engine horsepower=traveling horsepower+working horsepower+fan horsepower.
When work is done by the bulldozer, the traveling horsepower occupies a large percentage of the engine horsepower, while the working horsepower occupies a small percentage. Additionally, since the cooling fan is large in size, the working horsepower is low relative to the fan horsepower.
Consequently, the working horsepower is substantially negligible, and the relational expression above can be rewritten as follows:Engine horsepower=traveling horsepower+fan horsepower.
The engine mounted on a bulldozer is a diesel engine, and the engine output is controlled by adjusting the amount of fuel injected into a cylinder. The adjustment is performed by controlling a governor provided in the fuel injection pump of the engine.
A variable-speed governor is typically used as the governor to adjust the engine speed and the fuel injection amount according to the load so as to obtain a target speed according to an operational amount of a throttle dial or an accelerator pedal. This means that the governor controls the fuel injection amount so as to eliminate the difference between the target speed and actual engine speed.
FIG. 1 illustrates a relationship between engine speed Ne and engine torque Te, that is an engine's power curve (maximum torque line) R. The range defined by the engine's power curve R indicates the performance that is obtainable from the engine. The governor controls the engine to prevent the torque from surpassing the engine's power curve (maximum torque line) R to exceed the smoke limit and to cause the discharge of black smoke. The governor also controls the engine to prevent the engine speed Ne from surpassing the high idle speed NH to cause overspeed.
The traveling horsepower is obtained by subtracting the fan horsepower from the engine output.
On the other hand, the target temperature of the coolant is set to a temperature at which the optimum engine efficiency is obtained. The coolant temperature is varied by adjusting the cooling fan speed (fan speed) Nf. The coolant temperature is controlled at the target temperature by adjusting the fan speed Nf in accordance with an actual coolant temperature Tw. When the coolant temperature Tw is low, the fan speed Nf is decreased so that the coolant temperature Tw matches the target temperature. When the coolant temperature Tw is high, the fan speed Nf is increased so that the coolant temperature Tw matches the target temperature. The fan horsepower becomes higher as the fan speed Nf becomes higher.
As shown in FIGS. 2A, 2B, and 2C, the fan horsepower becomes higher (the range indicated by the oblique lines becomes greater) as the coolant temperature Tw becomes higher. In accordance therewith, the traveling horsepower is decreased and the tractive force is reduced.
In a bulldozer, as described above, output from a single engine is used both for the traveling horsepower and the fan horsepower. Therefore, the engine output that can be used as the tractive force is decided depending upon the value of the coolant temperature Tw, that is, the amount of the cooling fan load.
(Related Art Found in Patent Documents)
Conventionally, techniques have been known to control an engine according to the increase of fan horsepower to prevent the reduction of traveling horsepower (tractive force), as disclosed in Japanese Patent Application Laid-Open Nos. S62-178754 and 2003-161191, for example.
Japanese Patent Application Laid-Open No. S62-178754 discloses an invention according to which fan horsepower is calculated based on fan speed, and the engine is controlled according to the magnitude of the calculated fan horsepower so as to render the tractive force fixed.
Japanese Patent Application Laid-Open No. 2003-161191 discloses an invention according to which the engine is controlled according to the magnitude of load on a cooling fan or an air conditioner system compressor so as to increase the power of the diesel engine.
Additionally, Japanese Patent No. 2711833 discloses an invention relating to a hydraulic excavator, according to which the maximum absorption torque or capacity of a variable displacement hydraulic pump is varied according to various work modes so that the work is performed with the importance placed either on the amount of work or on the fuel economy.
According to the invention disclosed in Japanese Patent No. 2711833, the selection of a work mode enables the hydraulic excavator to be operated appropriately for placing importance on the amount of work, or on the fuel economy.
However, the invention of Japanese Patent No. 2711833 does not assume a bulldozer having a cooling fan that is driven by the engine. If the invention of Japanese Patent No. 2711833 is applied to such a bulldozer, the traveling horsepower and the working horsepower will vary according to the coolant temperature. This will possibly make it impossible to attain a desired amount of work and fuel economy envisaged for each of the work modes. If the coolant temperature is low, the traveling load will become high, and traveling load that is higher than the traveling load envisaged for each work mode will possibly be input to the traveling power train to produce excessive traveling horsepower. As a result, the durability of the traveling power train may be deteriorated.
According to the inventions disclosed in Japanese Patent Application Laid-Open Nos. S62-178754 and 2003-161191, the engine output that can be used as tractive force can be held fixed regardless of the coolant temperature or the magnitude of the cooling fan load. However, it is impossible to operate the work vehicle in accordance with the operating situation, for example with the importance placed on the amount of work, or with the importance placed on the fuel economy.
The present invention has been made in view of the circumstances described above. It is therefore an object of the present invention to enable a work vehicle having a cooling fan or any other auxiliary device driven by the engine to be operated, depending on operating situation, with the importance placed on the amount of work or on the fuel economy, and to ensure the durability of a traveling power train (or work machine drive equipment) regardless of which work mode is selected, by preventing the input of excessive traveling horsepower (or working horsepower).